Blood coagulation is a complex and dynamic biological process that depends on a series of interdependent biochemical reactions. Coagulation factor VIII (FVIII) is a key component of the blood coagulation cascade. When a bleed occurs, FVIII is directed to the bleeding site and forms a Xase complex with activated factor IX (FIXa) and factor X (FX). The Xase complex activates FX, which in turn activates prothrombin to thrombin, which activates other components in the coagulation cascade to generate a stable clot (reviewed in Saenko et al., Trends Cardiovasc. Med., 9:185-192 (1999); Lenting et al., Blood, 92:3983-3996 (1998)). In hemophilia A, a congenital X-linked bleeding disorder characterized by a deficiency in FVIII, the lack of functional FVIII hampers this positive feedback loop, resulting in incomplete coagulation, which manifests as bleeding episodes of increased duration rather than increased intensity (Zhang et al., Clinic. Rev. Allerg. Immunol., 37:114-124 (2009)).
The severity of hemophilia A varies based on the nature of any mutation to FVIII and the extent of function of any endogenous FVIII that is formed. About two thirds of patients have “severe” hemophilia, characterized as less than 1% functional FVIII. Patients with “moderate” hemophilia have about 1-5% functional FVIII, and patients with “mild” hemophilia have about 5-50% of normal FVIII function (Zhang et al., Clinic. Rev. Allerg. Immunol., 37:114-124 (2009)). Current treatment for hemophilia typically consists of factor VIII replacement therapy, in which the patient receives recombinant or plasma-derived factor VIII to prevent or treat bleeding episodes. However, in approximately 25-30% of patients with severe hemophilia A and in approximately 5% of patients with mild to moderate hemophilia A, inhibitory alloantibodies are produced against FVIII, abrogating the effectiveness of this treatment (Oldenburg and Pavlova, Haemophilia, 12 (suppl. 6):15-22 (2006)).
Inhibitor development is considered to be the most significant complication in the treatment of hemophilia. Patients with inhibitors have a higher mortality rate because their bleeding episodes become more difficult and costly to treat, and preventative treatment is generally not possible in these patients. In patients with high-titer inhibitors, there is an increased risk of developing recurrent bleeding in particular joints, which may ultimately result in decreased quality of life, disability, or death from excessive blood loss (Zhang et al., Clinic. Rev. Allerg. Immunol., 37:114-124 (2009); Gouw and van den Berg, Semin. Thromb. Hemost., 35:723-734 (2009)). Although in some cases immune tolerance induction can eradicate FVIII inhibitors in patients with hemophilia A, relapse is possible and not all patients reach immune tolerance (Wight et al., Haemophilia, 9:436-463 (2003)).
Accordingly, there is a need in the art for the identification of genetic markers that are associated with increased likelihood of developing inhibitory antibodies to FVIII and for corresponding methods of identifying the presence of these genetic markers in hemophiliac patients. Prior or early identification of such genetic markers could lead to dose and/or timing adjustments and to the use of alternative therapies to avoid the development of antibodies to FVIII.